Finding out your partner has cheated on you is never good news, but it can be especially painful if you felt you were in a happy relationship and had no reason to doubt they would be faithful.
But you shouldn't torture yourself with the idea that they were secretly unhappy, as this may not be the case.
"Even people in generally good quality relationships can be tempted to cheat," explains Relate counsellor Martin Burrow.
And with statistics revealing 20% of adults in the UK have had an affair and over 30% have considered cheating, it is clear that thinking of infidelity as only a problem for miserable couples would be wrong.
Sarah Ryan, director of matchmaking agency Simantov International, who has done extensive research in this area, agrees: "If someone has the inclination to go astray, no matter how happy a home they live in, they will do so."
Although this might all sound concerning, rest assured cheating never just comes "out of the blue" (even if it seems that way) and there is always a reason why people play away from home.
1. The cheat is not sure what they want for themselves.
It can be difficult to know what you want - even when you're perfectly happy with your partner, it might seem the grass is greener in singledom.
Burrow says: "Many of us seek security and stability which a long-term relationship can bring, whilst simultaneously longing for the adventure and excitement of a new partner. These two basic needs can feel conflicting and very confusing," - and this confusion can lead people to stray.
2. The cheat wants to be sure they are with the right person.
Your partner might be sure they are happy being in a relationship, rather than playing the field, but if they are romantically inexperienced, they may wonder whether they are with the right person.
"Romanticism would have us believe there's one ideal partner out there who will meet all our needs to the point when we'd never as much as look at another person," says Burrow.
"The reality is there's no such thing as the perfect partner or the perfect relationship, the idea of perfection is nothing more than that - an idea."
3. The cheat thinks all sexual attraction must lead to action.
Everyone knows it is very easy to present a flawless version of ourselves to the outside world, and this gets a little harder when we let our guard down and get close to people.
As a result, there are probably going to be days when you want to jump into bed with your partner less than an exciting stranger, but that doesn't mean you have to act on your impulses.
"It Is completely natural for us (and our partners) to find other people attractive in all sorts of ways, including sexually," says Burrow.
"Often we'll be attracted to somebody because they bring out an aspect of our personality that our partner doesn't. People may then start to worry that they aren't with the right person and can be tempted to cheat."
4. The cheat is separating sexual infidelity from love.
There are different types of cheating - emotional, physical, or even a long-term affair - but if your partner is 'just' having sex they might see what they are doing as separate to what the two of you share together.
Ryan says: "Some people have strong beliefs that sex and love are two things that are not intertwined and so in their mind being physical has nothing to do with the amount they are emotionally committed, hence, they enter into a sexual relationship with someone else outside of their existing relationship."
5. The cheat doesn't know differently.
Some people see cheating as a normal part of all relationships - a learned behaviour that they have copied from parents, family or past relationships.
Ryan says: "I don't plan to go down the whole Freudian route with this but if a cheater has ever seen parents or peers steer away from a committed relationship into a sexual or emotional affair with another then somehow it can be instilled in their mind that cheating is okay - they may have been conditioned to think that cheating is an acceptable part of life."
6. The cheat is bored.
People may succumb to the lure of something exciting and novel, even if they are otherwise happy.
"We live in a world where our senses are constantly stimulated, our attention spans have dwindled in a digital age to that less than goldfish of fewer than eight seconds,"says Ryan.
"I can only imagine how that affects the way we communicate in relationships and our desire to always want more, despite being in what to the casual onlooker seems to be a happy, healthy and committed relationship.
"If we are constantly seeking to be stimulated it makes sense that relationships will be affected and those with the inclination to cheat might feel compelled to do just that due to sheer boredom and perceived lack of thrill in their life."
7. The cheat just had an opportunity.
You might not want to hear it, but it seems that even in the happiest of relationships, sometimes people don't have enough willpower to resist an opportunity.
"Now, with the ease with which you can contact somebody you like via instant messaging, text or email, there's never been so much opportunity to cheat. Add alcohol to the mix and you've got a recipe for deceit," says Burrow.